practice/my_library/models/relation.py

from odoo import models, fields


class ChildModel1(models.Model):
    _name = 'child1.model'

    parent_id = fields.Many2one(comodel_name='parent1.model', string='Parent')


class ParentModel1(models.Model):
    _name = 'parent1.model'

    child_ids = fields.One2many(comodel_name='child1.model', inverse_name='parent_id', string='Children')


class ChildModel2(models.Model):
    _name = 'child2.model'

    parent_id = fields.Many2many(comodel_name='parent2.model', string='Parent')


class ParentModel2(models.Model):
    _name = 'parent2.model'

    child_ids = fields.Many2many(comodel_name='child2.model', string='Children')